As I understand it, Grant sent Butler to BH to scare Lee and cause him to rush back to Richmond to secure it. Grant would be in hot pursuit and would join Butler, taking control in the Richmond area. But Butler scared Lee, but did not hurt Lee -- Lee lost his script and did not run back to Richmond. This left Butler without direction, which Grant did not provide. Butler showed his weakness and accomplished nothing.
Had Butler been given an objective --- break the R&P RR and keep it broken or capture Petersburg -- he could have made a major difference in the war. But he did not attack Petersburg because he did not have orders to do such from Grant. Grant did not consider the possibility that Lee would not run and therefore did not give Butler the right objective.
I personally think that's being way way too generous to Butler. I know some (not you) want desperately to 'rehabilitate' him, but … no. No West Point course is going to discuss the military brilliance of his blunders. It's also a stretch to assume how much a CinC has to detail taking initiative and advantage of all local opportunities (and then be faulted for not noting all that might occur). "More opportunistic or more detailed" instructions would simply have been used by poltroons like Butler to excuse away their failures anyway - I had too many conflicting objectives, I technically did this and no more, so I met the letter of the text, etc etc.
This is what failed commanders always do. Nitpick a detail to excuse a whopping blunder. I can accept it would have been great had Grant been clairvoyant. It would have been great if Grant could write a legalese contract so bindingly tight that Butler could not excuse his failures through parsing and wordsmithing. But neither was possible, nor should we expect it to have been.
We have every right - as Grant did - to expect that meeting with a general, discussing with a general, emphasizing speed and multiple opportunities, providing seasoned help - would be enough for a competent general. Butler was incompetent, so it didn't help. In Butler's (weak) defense, the attempt to provide him with supporting generals backfired because of their inability to all coordinate and get along (Butler is responsible for that in some significant part as well). And we should give Butler props for his non-military Administrative abilities, as well as his surprisingly modern human rights outlook.
But sorry, I've seen the arguments ad nauseum excusing every military failure of Butler as "someone else's fault", every tentative micro-half-move being touted as a 'full accomplishment of his written objectives', every inaction and wrong action as 'justified by military necessity'. And from a military perspective, it's one big steaming heap of b*******.
Good generals win more than they lose, take risks, and make the best of their opportunities. Bad generals make excuses. Butler was a incredibly bad general, and blundered a golden opportunity. Nothing changes those facts, or excuses his actual performance.
Grant's biggest blunder was lacking the power to remove Butler and Sigel from command positions in the first place (I would have loved to see Butler kept in administration, and Sigel kept in recruiting, but politics are politics). Grant bowed to the required political assignments and did what he could with them - and both Sigel and Butler failed to even come close to reasonable performances.
Poltroons blame others for their failures. Great commanders win despite the failures of poltroons.
Basta ya!